The present invention relates to a method for automatically attaching a sub-assembly such as a vehicle part to a main assembly such as a vehicle body frame, and particularly to an improvement in the process of automatically attaching a sub-assembly, such as a strut assembly, to a predetermined location of the main assembly, such as a vehicle body frame, via sets of fasteners, such as bolts and nuts.
Conventionally, in an assembly line for automotive vehicles, a plurality of operators carry out manual attachment operations by which a strut assembly of a strut-type front suspension (hereinafter, referred to simply as a strut) is attached to a predetermined location on a car body frame (referred to simply as a vehicle body) mounted on a conveyer belt. In detail, one operator carries one of the struts stored in a bin toward the vehicle body, fits an upper positioning portion of the carried strut into a positioning hole provided in the vehicle body, and grasps the strut so as to hold it in place in the positioning hole of the vehicle body. Another operator then moves a fastening tool for a fastener such as a nut to the predetermined location on the vehicle body where nuts held by the fastening tool are tightened onto each bolt extending from the strut and penetrating through holes provided around the predetermined location on the vehicle body. In this way, the manual attachment operation of the strut to the vehicle body is completed. However, it is extremely troublesome for the operators to carry out the above-described attachment operation because of many disadvantages, e.g., the struts are relatively heavy. Therefore there is an increasing demand for automating attachment operations such as attaching struts to the vehicle body.
To meet this industrial demand, a method for automatically attaching struts to a vehicle body using industrial robots has been proposed.
In one such method, when the vehicle body transported along a conveyer belt reaches a predetermined location of a working station, the required strut is grasped by one robot and carried toward a predetermined location of the vehicle body and, at this time, an automatic fastening tool is moved to the predetermined position opposite the carried strut by means of another robot and the strut is automatically fastened to the vehicle body. In this case, each of the robots moves along a trajectory preset by a teaching method involving a dryrun manipulation of each robot or a teaching box so as to carry the strut or fastening tool to opposite sides of the predetermined location on the vehicle body.
However, since the accuracy of the stopped position of the vehicle body transported by the conveyer belt is relatively low due to the current control technology for conveyer belt systems, the working positions of both the strut and the fastening tool, carried by the respective robots instructed by means of the above teaching method, generally deviate slightly from the normal stopped position. Therefore, although it is necessary to carry the strut to a position appropriate for attaching the strut to the vehicle body and also to locate the fastening tool appropriately for fixedly attaching the strut to the vehicle body, it requires a lot of time for the respective robots to accurately position the strut and the fastening tool separately opposite the predetermined location of the vehicle body. Consequently, such an automation method as described above will not minimize the total operation time required to automatically attach the strut to the vehicle body. In addition, if the respective robots are adapted to separately position the strut and the fastening tool opposite the predetermined position of the vehicle body, a control unit storing pre-programmed instructions by which the respective robots carry the strut and the fastening tool to their respective given positions opposite the predetermined location of the vehicle body is additionally required in order to correct for the positioning error of the vehicle body so that the overall system for automatically attaching the strut to the vehicle body becomes complicated.
With these problems in mind, the Applicants have already filed previously an application for patent in U.S. as Ser. No. 534,241 and EPO as Ser. No. 83109404.0 entitled "METHOD AND SYSTEM FOR AUTOMATICALLY ATTACHING SUB-ASSEMBLY TO MAIN ASSEMBLY USING INDUSTRIAL ROBOTS", both filed 21th Sept. 1983. The system disclosed therein will be briefly described below. A vehicle body is transported along a conveyor belt and is stopped at the predetermined location of an attaching station. Thereupon, one of the struts stored in a bin is grasped and carried by one robot toward the vehicle body stopped at the station, while the fastening tool is carried toward the vehicle body by means of another robot. In this situation, the strut is automatically attached to the vehicle body by means of fasteners. However, since the strut is not fixed to the vehicle body when the strut is to be positioned opposite the predetermined position on the vehicle body, the one of the two robots which has carried the strut to the vehicle body needs to support the positioned strut until a adequate number of fasteners have been completely tightened onto the opposing fasteners provided on the strut by means of a fastening tool.
Therefore, the free working space of the other robut which carries the fastener fastening tool to the predetermined location on the vehicle body is limited by the first robot supporting the strut at the predetermined location on the vehicle body. Consequently, the programming for the second robot through the teaching method becomes complicated.